Previous analysis of the virion proteins of an N- and a B-tropic type C virus of BALB/c mice, of 16 N-tropic recombinants (XLPN viruses) between these viruses, and of eight NB-tropic viruses derived from the B-tropic virus suggested that among these closely related viruses N-, B-, or NB-tropism was associated with the electrophoretic mobility of p30 on sodium dodecyl sulfate-polyacrylamide gels, and thus that p30 might determine this phenotype. To obtain further evidence for the association of structural markers of p30 with N-, B-, or NB-tropism, we have analyzed the p30's of these same viruses by using two-dimensional tryptic peptide mapping and slab gel isoelectric focusing. The results of these analyses suggest that (i) a single peptide unique to the N-tropic virus p30- is present in the p30 of all N-tropic recombinants; (ii) a single peptide unique to the B virus p30 is not present in p30's of the N-tropic recombinants, and this peptide is also absent in p30's of NB-tropic viruses derived from the B-tropic virus; and (iii) p30's of NB-tropic viruses possess a new tryptic peptide not found in the p30 of their B-tropic virus progenitors, and this new peptide is not found in the p30 of the N-tropic virus of BALB/c or the XLPN viruses. These results are consistent with the possibility that p30 may determine the N-, B-, or NB-tropism of murine leukemia viruses. In addition, these studies indicate that some of the N-tropic recombinants have experienced recombination within the p30 gene.